1. Field of the Invention
The present invention is related to speed measurement technology, and more particularly, to measuring vehicle speed based on movements of a video camera located on a moving vehicle.
2. Description of the Related Art
Navigation based on correlation of extremes is method for measuring vehicle speed relative to an underlying surface. An image of the surface produced by an analog or digital video camera is used as initial information. The video camera is pointed at the surface. The video camera transforms an optical image of the surface projected onto a video matrix into a sequence of digitized image frames. An off-set between the adjacent frames having maximally coinciding images is found.
The information about the time of shooting of each frame allows measuring the speed of movement of the image on the video matrix (sensor). A value of height of location of the video camera relative to the underlying surface allows for calculating the speed of the video camera relative to the surface based the measured speed of the image.
A speed measuring method is disclosed in U.S. Pat. No. 4,162,509. This method uses wireless matrix-based speed measuring device with two optical units. A time interval between image generations by these optical devices is controlled. The speed is calculated by dividing a distance between the optical devices over the controlled time interval. The main disadvantage of this correlation-based method is its low accuracy when the movement is slow or is performed in a reverse direction.
A method for measuring speed of vehicle is disclosed in U.S. Pat. No. 5,734,337. According to this method, changes in positioning of the vehicle in the adjacent frames taking into account the camera angle are used for speed calculation. The camera angle is used for compensation of perspective distortions. The disadvantage of this method is that the camera is firmly fixated on a static platform and the camera angles do not change.
Another speed measuring method is disclosed in U.S. Pat. No. 4,495,589. This method is used for measuring aircraft speed relative to the earth surface. The speed is measured by calculating the off-set in earth surface profile for adjacent image frames taken by the same video camera. This method produces inaccurate measurements when a camera is not oriented exactly perpendicular to the surface due to perspective projection distortions. When a camera is not perpendicular to the surface (i.e., it has some inclination), the image of the surface has a perspective projection distortion—http**en.wikipedia.org/wiki/Perspective_projection_distortion.
It is apparent that improved method for determining speed of the camera is desired. Accordingly, there is a need in the art for a system and method that addresses the need for efficient and precise determination of camera and vehicle speed.